Providing effective and reliable communication between multiple terminals through satellites is important.
This invention relates to satellite communication networks particularly for passing information from a hub terminal to ensure control and regulation of remote equipment.
Problems related to known satellite communication systems include the following:
(1) A concern for critical applications using satellite communications includes the loss of communication due to failure of the satellite transponder system. Typically, such systems incorporate redundancy by means of terrestrial communications or dual implementation of the earth station terminals at each site combined with the use of a second satellite transponder. These approaches are not economically viable for satellite communication systems involving numerous, unmanned remote terminals.
This problem has limited the use of satellite-based communications in critical applications such as Supervisory, Control And Data Acquisition (SCADA) networks. SCADA networks are used for example by electric utilities; to issue control commands and collect operational data from electric generation and distribution equipment. The network may contain a thousand or more remote terminals. While failure of a single remote terminal node is tolerable, failure of the satellite equipment in a nonredundant network is unacceptable. This would result in the total loss of command and control for the long period of time required to retune each remote terminal to a new transponder frequency and reset the antenna position to align with the new satellite.
In addition to equipment failures, loss of communication occurs when the sun lies on the line of site formed by the location of the earth terminal and the satellite. The sun's radiation causes the terminal's receiver noise level to increase and interfere with reception of the satellite's signal. The sun's adverse impact on signal reception is greatest for hub terminals which typically employ a high gain antenna, i.e. beamwidth less than one degree. Presently, communication systems which cannot tolerate loss of communication due to the sun, utilize two hub terminals, geographically separated by typically 1,000 miles or more, in order that the line-of-side to the satellite be sufficiently different for each hub terminal such that the sun will only interfere with one hub terminal at a time. The geographical separation necessitates an additional communication link to bring the data to the desired centralized facility. The redundant hub at the same site only requires a second antenna and not duplicate electronics.
(2) The time required to acquire the satellite signal and commence useful communications is prohibitively long for mobile terminals employing spread spectrum coded signals and moderate to high antenna gains. Spread spectrum is used to artificially increase the bandwidth of the transmitted signal relative to the bandwidth required to transmit the information. This technique reduces the power spectral density i.e. power per unit bandwidth of the transmitted signal. The FCC regulates the maximum allowable spectral density utilized by each satellite communication system in order to control the interference imposed on other systems. Non-spread spectrum systems utilize the transmissions of symbol bits which are related to the information data. The direct sequence approach to spread spectrum entails subdividing each symbol bit into a pseudo random sequence of bits, termed chips. The polarity of the sequence, hence the chips, is determined by the polarity of the symbol bit. The receiver can de-spread the transmitted signal, i.e. remove the pseudo random sequence, provided it has a apriori knowledge of the sequence. The de-spread process consists of the cross-correlation of the known sequence with the received signal. The signal is extracted only when known sequence is aligned with the sequence embedded with the signal. The alignment of the sequence is termed code phase.
A typical antenna for a mobile terminal provides a 4.degree. azimuth beamwidth and a broad elevation beamwidth encompassing the total range of expected elevation angles to the satellite. To cover 360.degree. of azimuth requires a minimum of 90 azimuth positions. During acquisition, these azimuth positions must be scanned to ascertain the presence of the satellite signal. At each azimuth position, the receiver frequency needs to be stepped and searched to ascertain the correct setting. The frequency search is required to compensate for doppler shift and inaccuracies in the local frequency reference of the mobile terminal. Depending on the terminal design, approximately 50 frequency steps are usually required.
Finally, at each antenna azimuth position and frequency setting, the spread spectrum signal must be scanned to establish the correct code phase. For a typical code length of 255 chips, this entails testing 255 phases. Signal acquisition involves a three dimensional search i.e. azimuth, frequency, and code phase. For the terminal described, there are 1,147,500 (90.times.50.times.255) possible combinations. At a nominal time of one millisecond to test one combination, the acquisition process could take 1,147 seconds. For most applications, this time is too long.
Presently mobile systems rely either on the transmission of sufficient power to allow the use of an antenna with a gain below 6 dB and a sufficiently broadbeam antenna pattern as not to require a directional search. Alternatively, a time division multiple access is used in combination with a moderate gain antenna with a search in azimuth and frequency, but not in code phase.
These systems require power spectral densities that exceed the standard FCC regulations regarding fixed-site satellite communication systems. Accordingly, special licensing as a mobile system is required.
(3) For SCADA networks and other satellite communication systems, it is desirable to provide a time distribution function at approximately one microsecond resolution in addition to the two-way data transfers. Up to the present, the time distribution resolution has been determined by the data communication rate. For low data rate systems, the time resolution has been inadequate. For example, at 9.2 kbps the nominal resolution will be worse than 100 microseconds. A satellite communication system is needed to provide time distribution at high resolution independent of the data rate.
(4) Code multiplexing, is presently inefficient when used for transfer of short bursts of data. Code multiplexing is an extension of spread spectrum technology. For direct sequence, spread spectrum, multiple signals are transmitted at the same time and at the same frequency, but using different code sequences. The de-spread process, used by the receiver, rejects all signals except the one with the matching code. Multiple de-spread processes may be performed using each of the expected code sequences to extract all the signals.
Typical SCADA networks require one second transmissions from each remote terminal. De-spread of code multiplex signals requires a nominal 0.25 seconds for code phase and frequency determination. This results in a 25% loss in useful utilization of the communication link.
Code acquisition time is currently reduced through the use of special large scale integrated circuits called spread spectrum matched filters. These circuits can establish the correct code phase within the receipt of one complete code sequence. Although these circuits do provide rapid code phase acquisition, they involve complex circuitry and are expensive. Also the frequency acquisition process remains unimproved. Elimination of the time to acquire the spread spectrum code phase and frequency would significantly enhance the use of code multiplexing.
(5) A limitation of code multiplexing is the inability to demodulate one or several relatively weak signals in the presence of multiple, comparatively strong signals. This results from the non-zero cross-correlation between the spread spectrum encoded signals. The strong signals have the effect of increasing the apparent noise level to a value too high for processing the weak signal.
The reduction in signal level results from many causes including attenuation due to heavy precipitation. For SCADA networks, the transmissions from a remote terminal located within a thunderstorm would be masked by transmissions from remote terminals in clear weather. In this case, the most important data could be lost.
A method of dealing with this problem has been the incorporation of power equalization circuitry. In such systems, the terminal monitors the strength of received signals, either its own signal returned by the satellite or another terminal's signal. The transmit power is adjusted based on the signal strength. This is based on the assumption that all variations in signal strength are due to atmospheric and weather conditions. This approach involves significant increases in terminal complexity and cost.
Recent efforts have focused on the development of spread spectrum cedes producing lower cross-correlations. This should provide a moderate increase in the dynamic range of received signals with only a small increase in cost and complexity.
A system architecture and processing methodology is needed which substantially enhances the ability to process code multiplexed signals of widely varying amplitude.
(6) SCADA networks typically utilize a polling method to determine which remote terminals are to transmit. Two polling methods are used: one is a fixed sequence method, and the other method is real-time control.
In the filmed sequence method, each remote terminal is assigned a time slot. The time slots may be altered by means of commands transmitted by the hub terminal. The advantage of the fixed sequence method is the amount of data required to be transmitted by the hub terminal to the remote terminals is low provided that the established polling sequence is not altered. In order to retain the benefits of low data rate transmissions by the hub terminal, a long period of time is required to implement significant changes to the polling sequence. For this reason, the fixed sequence method is not suitable for networks requiring rapid and numerous variations in the polling sequence. Such networks must use the real-time control polling method.
By this method, the hub terminal transmits a command to each remote terminal to request the remote terminal to transmit data. This requires the transmission of a large amount of data by the hub terminal with an attendant increase in network costs.
A polling method is needed which achieves rapid and numerous changes to the polling sequence yet does not impose a high transmit data rate requirement on the hub terminal.
In general, a disadvantage of the polling method is the occurrence of an emergency or high priority event at a remote terminal will not be transmitted to the hub terminal until requested by the hub terminal by the normal polling sequence. A single catastrophic event may cause an emergency situation to exist simultaneously at numerous remote terminal sites.
A method is needed to allow remote terminals to immediately notify the hub terminal of a local emergency condition and to prevent numerous simultaneous notifications from interfering with one another.
The polling method, used by SCADA networks, retrieves data on a periodic basis at an interval too long to enable detailed analysis of intervening transient events. Continuously recording data, at the rapid rate suitable for detailed analyses, between network polls is cost prohibitive since the quantity of data required for detailed analysis is an order of magnitude higher than for normal SCADA operation. For example, detailed analysis of an electrical transient would require a measurement every 100 microseconds at 12 bit resolution starting one second before the event and continuing through one second after the event. Continuous recording and retrieval of this data requires a 120,000 bps network throughput allocated to this individual remote terminal. This exceeds the data rate capacity of typical SCADA networks which are limited to 10,000 bps or less per remote terminal.
A method is needed which provides the rapid measurements associated with detailed transient analyses, yet maintains the average data rate within the capacity of conventional SCADA networks.
(7) The installation of numerous remote terminals is presently expensive and time consuming. The antenna of the remote terminal needs to be aligned to the direction of the satellite. Coarse antenna alignment is accomplished using surveying equipment. This is followed by fine adjustments; to maximize signal strength. Cables need to be constructed, routed and secured to provide a data interface between site equipment and the remote terminal. An equivalent cabling process is needed to bring electrical power to the remote terminal.
A remote terminal method is needed which eliminates or automates the installation process.